The invention relates to a system for reading magnetic information, comprising:
a read head provided with a magnetoresistive bar whose electric resistivity varies when it is subjected to a magnetic field variation, said bar having a first and a second terminal and being intended to be traversed by a bias current of a predetermined value, PA1 a second amplifier comprising a third and a fourth transistor arranged as a differential pair, with their bases connected to the first and the second terminal, respectively, of the magnetoresistive bar and at least one collector constituting an output of the second amplifier, PA1 a first and a second capacitor having equal nominal values, the first capacitor being arranged between the base of the first transistor and the first terminal of the magnetoresistive bar, the second capacitor being arranged between the base of the fourth transistor and the second terminal of the magnetoresistive bar, and PA1 an analog adder having a first input connected to the output of the first amplifier, a second input connected to the output of the second amplifier and an output intended to supply a signal which is representative of the sum of the values of the signals received at the first and second inputs. PA1 a first and a second module of the transconductance type, each having an input constituting one of the inputs of the adder and an output intended to supply a current which is proportional to the value of a voltage received at its input, and PA1 a resistor having one terminal connected to a fixed voltage terminal and another terminal connected to the outputs of the first and second transconductance modules, the voltage at the terminals of said resistor constituting the output signal of the analog adder. PA1 a pair of transistors, referred to as primary pair, whose emitters are connected to a current source, whose bases are connected to the collectors of the transistors of the amplifier, and whose collectors are interconnected via a capacitor, and PA1 another pair of transistors whose emitters are connected to a current source, whose bases are connected to the collectors of the transistors of the primary pair, and whose collectors are connected to the bases of the transistors of the amplifier.
a first amplifier comprising a first and a second transistor arranged as a differential pair, with their bases connected to the first and the second terminal, respectively, of the magnetoresistive bar and at least one collector constituting an output of the first amplifier.
Such a reading system is described in the article "Preparing the preamplifier for the brave new world of MR heads", published in DATA STORAGE in April 1996. In this reading system, a coupling capacitor ensures, within the amplifier, a decoupling of the DC component of the signal which is present at the terminals of the magnetoresistive bar. This coupling capacitor is arranged between the emitters of the first and second transistors. The first amplifier has a passband which is limited by a low cut-off frequency and a high cut-off frequency, which passband must be as large as possible. Here, the low cut-off frequency is, in a first approximation, of the order of the inverse value of the product between the value of the coupling capacitor and the value of the resistance, viewed from the emitters of the transistors. As this resistance typically has a value of the order of about ten Ohms, a considerable value is to be given to the coupling capacitor for obtaining an acceptable value of the low cut-off frequency. If it is desired to obtain a low cut-off frequency FO of the order of megahertz values, the coupling capacitor must have a value of the order of about ten nanofarad, which is very difficult to realize in an integrated form. The use of an external capacitor induces the creation of parasitic inductances and resistances which are difficult to control and are detrimental to the satisfactory operation of the system, particularly at high frequencies, and also disturb the symmetry of the amplifier, which degrades its common-mode reject rate.
It is possible to replace the coupling capacitor mentioned above by two capacitors, referred to as decoupling capacitors, each arranged between the base of one of the transistors and one of the terminals of the magnetoresistive bar. In such an amplifier, the low cut-off frequency will be, in a first approximation, of the order of the inverse value of the product between the equivalent value of the two series-arranged decoupling capacitors and the base resistances of the transistors. As such a resistance is of the order of a hundred times larger than the resistance viewed from the emitters, the value of the decoupling capacitors used in such an amplifier may be, for the same low cut-off frequency FO of the order of megahertz values, of the order of a hundred times smaller than that of the coupling capacitor present in the known reading system. This leads to a value of the order of about a hundred picofarad, which remains difficult to realize in an integrated form.